The invention relates generally to inspection systems and more specifically to electromagnetic acoustic transducers for use in ultrasonic inspection systems.
Ultrasonic inspection is a commonly used non-destructive evaluation (NDE) technique for detecting corrosion and cracking in different objects such as metallic components, pipes, and so forth. Electromagnetic acoustic transducers (EMATs) are ultrasonic transducers that couple acoustic energy into and out of an object electromagnetically rather than through the use of an acoustic coupling medium such as water.
The absence of a coupling medium enables electromagnetic acoustic transducers to be used in a variety of applications where the use of the coupling medium between the transducer and the specimen is either impractical or undesirable. For example, if the specimen is coated with an acoustically attenuating material, acoustic wave propagation is not feasible. The absence of the coupling medium also improves reliability of the scanning process and hence reduces the risk of having to perform repeated inspections in cases where the coupling medium has been lost. The use of EMATs enables inspection at elevated temperatures, on moving objects, in vacuum, on oily or rough surfaces and also in remote locations.
An EMAT generates ultrasonic energy in an object by applying a magnetic field to the object using a magnet, and inducing radio frequency (RF) eddy-currents into the object using an RF coil. The RF pulses interact with the magnetic field to produce a Lorentz force, which in turn produces ultrasonic waves at the radio frequency in the object. The strength of the generated force depends on the proximity of the probe to the object.
A wear plate is commonly attached to the electromagnetic acoustic transducer to protect the magnets and the RF coil from wear due to the motion of EMAT and contact with other components, such as the objects being inspected. Typically, the wear plate is made of protective material layers. Such materials are usually electrically non conductive and non-ferromagnetic. For example, certain EMATs use a ceramic wear plate. The wear plate is usually disposed between an inspecting region on the object and the EMAT. Since the wear plates are made using materials that are neither electrically conductive nor ferromagnetic, the wear plates introduce higher reluctance paths between the active part of the EMAT and the inspected region. Such reluctance paths cause flux leakage between the poles of the magnet, which in turn reduces the strength of the magnetic field in the object.
One way to minimize the reluctance path is by using wear plates having relatively thin protective layers. One disadvantage with using thin layer wear plates is that the lifetime of the EMAT is significantly reduced. Increasing the thickness of the protective layer of the wear plate on the other hand, results in a decline of the force generated by the EMAT. Additional lift-off from the surface causes the force generation to be much weaker and hence degrades the inspection signals.
Thus, there is a need for a wear plate for an electromagnetic transducer that can retain the electromagnetic force while minimizing flux leakage.